Present clinical series show a 30 percent or less three year patency rate for small caliber, low flow synthetic arterial substitutes. We propose to test the possible improvement in graft patency of lining small caliber synthetic grafts with endothelial cells (E.C.) in a primate model (baboon), whose coagulation system is similar to man. Since E.C. inhibit platelet aggregation, lining synthetic grafts with E.C. should lower the inherent thrombogenicity of the flow surface. Existing research with E.C.-lined synthetic grafts has been restricted to larger caliber, high flow vessels in canine models. Moreover, these synthetic grafts were seeded with E.C. during the pre-clotting technique and immediately implanted so that the synthetic grafts did not have significant E.C. coverage during the first seven days - a critical period for acute graft occlusion. An alternative technique of seeding E.C. on synthetic grafts by growth in tissue culture and delayed implantation is hampered by optimal endothelial cell-synthetic graft adherence. We plan to assess in vivo the thrombogenicity of E.C.-lined Dacron grafts by the use of 111 In labeled platelets. The adherence properties of the E.C. lining will be determined by 113 M In labeling of E.C. Our previous studies with this primate small caliber graft model has shown that the acute patency rate (0-3- days) of an unseeded Dacron graft is 50 percent and it has a ten fold greater amount of 111 In platelet associated activity than arterial autografts. The E.C.-seeded grafts will be harvested subsequently for morphologic and functional studies of E.C. activity. Concurrent in vitro studies will: 1) explore chomotactic methods for improving E.C. motility and E.C.-graft adherence in order to insure earlier confluent coverage of the flow surface, and 2) examine complement mediated interactions of white blook cells with E.C. in order to improve E.C. viability. The results of these in vitro studies will be applied to the second phase in vivo studies where E.C. seeded grafts will be impregnated with chemotactic substances. An alternative group of animals will be treated with complement-depleting or white blood cell suppressing agents. These pharmacologic manipulations may improve endothelial cell viability and hence synthetic graft patency.